Infrared (IR) spectrometry is an excellent diagnostic tool for identifying, or lack of, a carbonyl functional group(s). Carbonyl stretching vibration absorbs between 1900-1600 cm-1—a region where few other functional groups absorb. In addition, the carbonyl vibration is typically intense and thus easy to spot.
The gas phase FT-IR spectra, shown below, represent five carbonyl bands differing in the neighbouring atoms: acid halide, carboxylic acid, ester, ketone and amide. The general pattern can be used to suggest a carbonyl group based on the wavenumber.
TIP: conjugated carbonyl groups and hydrogen bonding will typically shift the C=O band to a lower frequency. Ring strain and heteroatoms will shift the C=O band to a higher frequency.